check for MessageBox since only that class has the method isCloseable(), fixes #4342

Closes #4342

Merge request studip/studip!3146

lib/classes/PageLayout.php

@@ -604,7 +604,7 @@ class PageLayout
             'type'      => $message->class,
             'message'   => $message->message,
             'details'   => $message->details,
-            'closeable' => $message->isCloseable()
+            'closeable' => $message instanceof MessageBox ? $message->isCloseable() : false,
         ];
 
         if ($id === null) {